1. Field of the Invention
The present invention relates to a magneto-optical member which is used in a 45 degree Faraday rotator as one constituent member of an optical isolator for use in an optical fiber communication system, an optical measurement system and the like, and also to an optical isolator using the magneto-optical member. Now, the optical isolator includes a polarizer, an analyzer, a 45 degree Faraday rotator which is interposed between the polarizer and the analyzer and which has a magneto-optical effect such as the Faraday effect, and a magnet for applying magnetic field to the 45 degree Faraday rotator, and serves to propagate light emitted from a light source (semiconductor laser) to a transmission path such as an optical fiber or the like without any loss, and to block reflected light from the optical fiber or the like so as to prohibit the reflected light from returning to the light source (semiconductor laser).
2. Description of the Related Art
In an optical fiber communication system having a semiconductor laser as a light source, in particular, an optical system based on a high speed digital transmission or an analog direct modulation mode, if reflected light from optical connector connections, optical circuit components and the like which are used in an optical fiber circuit returns to the semiconductor laser or an optical amplifier, it becomes difficult to maintain high quality transmission due to degradation of frequency characteristics or generation of noises. An optical isolator is used for the purpose of removing the reflected light.
FIG. 7 shows the construction of an optical isolator, and FIG. 8 shows the principles of operation thereof.
In FIGS. 7 and 8, an optical isolator 1 is generally constituted by a polarizer 2A and an analyzer 2B which allow only the light component having a fixed plane of polarization to pass through, a 45 degree Faraday rotator 3 which is provided between the polarizer 2A and the analyzer 2B and rotates the plane of polarization of light by 45 degrees, and a permanent magnet 4 for applying a magnetic field H to the 45 degree Faraday rotator 3.
Light 301 which travels in the forward direction shown in a part (I) of FIG. 8 is not polarized, but after having passed through the polarizer 2A, becomes light 302 which has only a component oriented in the direction of the polarization of the polarizer 2A. Then, after having passed through the 45 degree Faraday rotator 3, the light 302 becomes light 303 in which the polarization direction is rotated by 45 degrees. If the polarization direction of the analyzer 2B is adjusted so as to run parallel to the polarization direction of the light which has been rotated by 45 degrees, the light passes through the analyzer 2B with a minimum loss.
On the other hand, as shown in a part (II) of FIG. 8, as for light 305 which has been reflected and propagates from the optical fiber or the like in the backward direction, only light 306 having a component oriented in the polarization direction of the analyzer 2B passes through the analyzer 2B to be made incident on the 45 degree Faraday rotator 3 in the backward direction. This light 306 is further rotated by 45 degrees in the same direction as when traveling in the forward direction on the basis of the non-reciprocity peculiar to the Faraday effect. As a result, after having passed through the 45 degree Faraday rotator 3, the light 306 becomes light 307 which has a polarization direction perpendicular to the polarization direction of the polarizer 2A and therefore is blocked at the polarizer 2A so as not to return to the light source.
Of the constituent members of the optical isolator 1, the 45 degree Faraday rotator 3 has a large influence on the performance of the optical isolator 1. The 45 degree Faraday rotator 3 is required to have a small element length necessary for rotating the plane of polarization by 45 degrees and a large light transmittance. Up to now, a magneto-optical member which is employed as the 45 degree Faraday rotator 3 may be made, for example, of a yttrium iron garnet (YIG) bulk single crystal (about 2 mm in thickness), or of a bismuth-substituted rare earth iron garnet (BiYIG) thick film single crystal in which a part of yttrium is substituted with bismuth having a large magneto-optical performance index (several hundred xcexcm in thickness). Recently, the BIYIG thick film single crystal is employed as the magneto-optical member in many cases because it is advantageous in the miniaturization of the optical isolator.
This BiYIG thick film single crystal is produced by utilizing a liquid phase epitaxial (LPE) growth method. In order to carry out a stable liquid phase epitaxial growth, many production parameters need to be accurately controlled, and therefore it is difficult to grow uniform single crystals over a large area with a high yield. In addition, the fact that it takes 20 hours or more to grow the single crystals and that an expensive non-magnetic GGG (gadolinium gallium garnet) single crystalline wafer needs to be employed for the substrate is an obstacle to the reduction in cost.
Under the situation described above, in order to solve the above-mentioned problems associated with the magneto-optical member which is produced by utilizing the liquid phase epitaxial growth method, the present inventors disclosed in Japanese Patent Application No. Hei 11-283512 the magneto-optical member (45 degree Faraday rotator) made of one-dimensional magnetic photonic crystal which causes the enhancement of the magneto-optical effect (the Faraday effect is one kind of magneto-optical effect) due to the localization of light. Then, though the above-mentioned magneto-optical member made of the one-dimensional magnetic photonic crystal is of polycrystals with a thickness of several xcexcm, a large Faraday rotation angle can be obtained.
In this connection, the one-dimensional magnetic photonic crystal is also described in JOURNAL OF THE MAGNETICS SOCIETY OF JAPAN, Vol. 23, pp. 1861 to 1866 (1999). The one-dimensional magnetic photonic crystal is structured such that a layer of magnetic substance and a layer of dielectric substance are alternately and multiply laminated with each thickness thereof irregular and formed into a thin film, or that two dielectric films, which are each formed of multiple layers having two kinds of dielectric substances different from each other and alternately laminated with each thickness thereof regular, sandwich a layer of magnetic substance having a thickness different from the layers of dielectric substance.
Of the above-mentioned structures, the latter is identical to the structure known long as the Fabry-Perot resonator structure and is found to be easily manufactured and at the same time to have a large enhancement as well. A magneto-optical member 10 made of one-dimensional magnetic photonic crystal is shown in FIG. 9 as an example.
The magneto-optical member 10 made of one-dimensional magnetic photonic crystal includes two periodic dielectric multilayer films 13 and 14 in each of which two kinds of dielectric substances (dielectric thin films) 11 and 12 are alternately laminated with each thickness thereof regular, and a magnet-optical thin film (made of magnetic substance) 15 which is provided between the two periodic dielectric multilayer films 13 and 14.
The periodic dielectric multilayer films 13 and 14 play a part of the reflecting mirror of the Fabry-Perot resonator. While a (Ta2O5/SiO2) system multilayer film is generally employed as the periodic dielectric multilayer films 13 and 14, a (Si/SiO2) system multilayer film has also been proposed in which a large Faraday rotation angle can be obtained with a smaller number of laminations than in the (Ta2O5/SiO2) system multilayer film. The thickness of each of the dielectric substances (dielectric thin films) 11 and 12 needs to be designed in such a way that its optical length (actual thicknessxc3x97refractive index) is equal to xcex/4 (xcex is the wavelength of light). In addition, it is general that the optical length of the magneto-optical thin film 15 in which the localization of light occurs is set equal to mxcex/2 (m is a positive integer).
Now, FIG. 10 shows the dependency of the light transmittance and the Faraday rotation angle on the number of laminations in one-dimensional magnetic photonic crystal (the magneto-optical member 10) of (Ta2O5/SiO2)n/BiYIG/(SiO2/Ta2O5)n structure (the number n of laminations=6, 7 and 8) as one example of the (Ta2O5/SiO2) system. FIG. 11 shows the dependency of the light transmittance and the Faraday rotation angle on the number of laminations in one-dimensional magnetic photonic crystal (the magneto-optical member 10) of (Si/SiO2)n/BiYIG/(SiO2/Si)n structure (the number n of laminations=3, 4 and 5) as one example of the (Si/SiO2) system. The wavelength of light is 1.3 xcexcm and the optical length of the magneto-optical thin film (magnetic substance) 15 is xcex/2.
As apparent from FIGS. 10 and 11, both in the (Ta2O5/SiO2) system multilayer and the (Si/SiO2) system multilayer film, the Faraday rotation angle increases in proportion to the increase in the number n of laminations and the light transmittance decreases largely with the increase in the Faraday rotation angle. The decrease of the light transmittance means that the transmittable distance of laser beam which propagates through the optical fiber is shortened. This constitutes a serious obstacle to the construction of the system.
The present invention has been made in the light of the foregoing, and it is therefore an object of the present invention to provide a magneto-optical member capable of ensuring an excellent light transmittance, and an optical isolator using the same.
According to a first aspect of the present invention, there is provided a magneto-optical member in which a plurality of one-dimensional magnetic photonic crystals, each having two dielectric multilayer films in each of which two kinds of dielectric thin films having optical characteristics different from each other are alternately laminated with each thickness thereof regular and a magnetic thin film which is provided between the two dielectric multilayer films, are laminated with two adjacent ones sandwiching a dielectric thin film having an optical length predetermined as xcex/4+mxcex/2 where xcex is a wavelength of light and m is 0 or a positive integer.
According to a second aspect of the present invention, in the first aspect of the present invention, an optical length of each of the two kinds of dielectric thin films is set to xcex/4.
According to a third aspect of the present invention, in the first or second aspect of the present invention, the two kinds of dielectric thin films have different refractive indexes from each other, and one dielectric thin film thereof having a smaller refractive index is in contact with the magnetic thin film.
According to a fourth aspect of the present invention, in any one of the first to third aspects of the present invention, an optical length of the magnetic thin film is set to an integral multiple of xcex/2.
According to a fifth aspect of the present invention, in any one of the first to third aspects of the present invention, two of the one-dimensional magnetic photonic crystals are provided, and the magnetic thin films thereof have respective optical lengths which are integral multiples of xcex/2 and different from each other.
According to a sixth aspect of the present invention, in any one of the first to fifth aspects of the present invention, the two kinds of dielectric thin films have different refractive indexes from each other, and the dielectric thin film having a predetermined optical length is made of a substance identical to that of the one dielectric thin film having a smaller refractive index out of the two kinds of dielectric thin films.
According to a seventh aspect of the present invention, there is provided an optical isolator in which the magneto-optical member defined in any one of the first to sixth aspects of the present invention is employed as a 45 degree Faraday rotator.